1. Field of the Invention
The present invention relates to a direct-current power supply, and in particular to a direct-current power supply which is powered by batteries.
2. Description of the Prior Art
In general, a secondary battery such as a Ni-Cd alkaline battery can be economically ulilized for a long time period due to its repeatedly rechargeable feature. Although the characteristic of the voltage supplied from a Ni-Cd alkaline battery does not significantly change while it is being used, its output voltage has a tendency to rapidly decrease around the time it has almost exhausted the electric charge accumulated therein. In contrast with such a secondary battery, although a primary battery has a lower unit price, it cannot be recharged; and as a result, the running cost required for employing it for apparatus is soared. The output voltage characteristic of such a primary battery is such that the output voltage develops a high value at the beginning of the battery utilization time period and it tends to proportionally decrease as the time elapses.
From an economical point of view, it is advantageous to employ Ni-Cd alkaline batteries in a portable camera, for example, a handy video camera or an electronic still camera. As described before, however, a Ni-Cd battery has the output voltage characteristic indicating an abrupt drop in the output voltage, which may lower at an unexpected time and may cause to lose a chance to shoot an important scene. In order to avoid such an disadvantageous accident, it will be advantageous to configure apparatus to include an emergency countermeasure for temporarily utilizing an ordinary primary battery, such as an alkaline-manganese battery.
For above-stated portable cameras, it is generally preferable to dispose therein alarm means for notifying beforehand that the voltage supplied from a battery has been lowered. The time for activating the alarm means is usually set in such a manner that the output voltage at the alarm point is slightly higher than the battery output voltage which is sufficiently low for allowing the normal operation of the apparatus. In such apparatus which allows both Ni-Cd alkaline batteries and alkaline-manganese batteries to be employed, however, there exists a remarkable discrepancy between the time periods required for these two types of batteries to reach the limit voltage after they are decreased to the alarm point for alarming the lowered output voltage condition.
Between a case in which Ni-Cd alkaline batteries are utilized to power an electronic still camera and a case in which alkaline-manganese batteries are employed for the same purpose, even if the same shooting condition is established in these cases, that is, even if the shutter is released under the same condition after the disk of camera has been driven into rotation at a predetermined rotating speed for a period of time, then there exists a difference of time period equivant to approximately 40 picture frames taken during the time period measured from when the alarm is generated in response to the battery voltage reaching the preset alarm voltage to when the limit voltage is reached in both cases.
This implies that, when alkaline-manganese batteries are temporarily employed for an electronic still camera designed to be operated by use of Ni-Cd alkaline batteries as its standard power source, 40 picture frames can be taken by the time when the limit voltage is reached after the alarm condition takes place. It can be readily imagined that the 40 frames are greater in number than required for nonprofessionals to shoot pictures.